Process for preparing sulfate-containing basic solutions of polyaluminumchloride

ABSTRACT

The process of preparing a solution of basic sulfate-containing polyaluminum chloride includes digesting an aluminum-containing substance with hydrochloric acid to form a resulting solution and an insoluble residue; filtering the resulting solution to separate the insoluble residue therefrom and to form a solids-free solution; concentrating the solids-free solution by evaporation to form a concentrated solution having an AlCl 3  content of about 30 percent by weight; recovering AlCl 3 ·6 H 2 O from the concentrated solution by crystallization; thermally decomposing the recovered AlCl 3 ·6 H 2 O by heating at 150 to 200° C. so as to form a solid basic aluminum chloride containing Al and OH in a molar ratio of OH/Al of 1.35:1 to 2.25:1; dissolving the solid basic aluminum chloride in an aqueous sulfuric acid solution and heat-treating at 40 ° C. to 70 ° C. for 1 to 3 hours to form a solution of basic sulfate-containing polyaluminum chloride having a sulfate content of 1 to 6 percent by weight, an aluminum concentration of 3 to 10 percent by weight and a molar ratio of OH/Al in the solution of the basic sulfate-containing polyaluminum chloride from 1.05:1 to 1.95:1.

DESCRIPTION

This invention relates to a process of preparing sulfate-containingbasic solutions of polyaluminumchloride. Such solutions are used asfloc- culating agents to the neutral aluminum salts, which have beenused for that purpose for a long time. Basic aluminum chlorides effect aflocculation and precipitation of the pollutants contained in the waterwithin a relatively wide pH range and the resulting flocs are so largethat they can easily be removed. The flocculation can be improvedfurther by the incorporation of polyvalent ions, preferably sulfateions, in the basic aluminum chloride because sulfate-containing basicaluminum chlorides will effect a faster hydrolysis than basic aluminumchlorides which are free of sulfates.

German Patent Publication 1,907,359 and German Patent Specification2,547,695 disclose processes of preparing sulfate-containing basicsolutions of aluminum chlorides. In said processes, sulfate ions areadded to basic solutions of aluminum chloride. In accordance with GermanPatent Publication 1,907,359 the basicity of the solutions is adjustedby an addition of calcium compounds and barium compounds. A disadvantageof the known processes resides in that the sulfate ions are precipitatedin part as CaSO₄ or BAO₄ and that said difficultly soluble compoundsmust be removed as by-products, which cannot be re-used. Besides, thesolutions prepared by the known processes tend to be unstable so thatdifficultly aluminum compounds are undesirably precipitated or saltedout and the flocculation activity is thus strongly decreased.

It is an object of the invention to provide for preparingsulfate-containing basic solutions of polyaluminumchloride a process bywhich products are produced which have a high flocculating activity andhigh stability whereas a formation of waste products is avoided.Besides, the basicity of the solutions should be adjustable within widelimits without an occurrence of instability.

The object underlying the invention is accomplished in thataluminum-containing substances are digested by a treatment withhydrochloric acid, the insoluble residue is filtered from the resultingsolution, the solids-free solution is subsequently concentrated byevaporation, AlCl₃.6H₂O is recovered from the concentrated solution bycrystallization, the AlCl₃.6H₂O is converted to a solid basic aluminumchloride by a thermal decomposition at 150 to 200° C., the solid basicaluminum chloride is charged into aqueous sulfuric acid and is dissolvedtherein with formation of Al(OH)_(x)Cl_(y)(SO₄)_(z) and the basicsolution of aluminum chloride is subsequently heat-treated at 40 to 70°C. for 1 to 3 hours.

The aluminum-containing substance which is used may consist, e.g., ofalumina hydrate and the hydrochloric acid used as a digestant contains15 to 35% HCl. The AlCl₃.6H₂O can be recovered by a crystallization inone or more stages and it is necessary to produce a crystallizate whichhas an optimum particle size for the subsequent thermal decompositionand a low residual moisture content. A centrifuge is desirably employedto separate the crystallizate from the mother liquor, which is recycledto the crystallizing process. The basic aluminum chloride which has beenproduced by the thermal decomposition is required to have a highsolubility and an optimum basicity (mole ratio of OH to Al). TheHCl-containing gases which have been liberated by the thermaldecomposition are reacted to form aqueous hydrochloric acid, which isre-used to digest the aluminum-containing substance. TheAl(OH)_(x)Cl_(y)(SO₄)_(z) formed as the basic aluminum chloridedissolved in aqueous sulfuric acid is present not only as monomers butalso as oligomers but the desired degree of polymerization is notachieved until the heat treatment.

The invention affords the advantage that raw materials are used whichconsist of aluminum-containing substances which have previously beendiscarded as waste material or had to be utilized with only a lowtechnological and economic benefit. Besides, the process in accordancewith the invention is an almost self-contained process, in whichsubstantially no by-products and/or waste products are formed.Specifically, the hydrochloric acid used to digest the raw materials isrecovered in part during the thermal decomposition of the AlCl₃.6H₂O andthe hydrochloric acid is desirably recycled. Besides the requiredbasicity is adjusted without a need for chemicals which would have to bedisposed of as waste products. Finally, the solutions prepared by theprocess in accordance with the invention have extremely good propertiesin use. This is particularly due to the production of a solid basicintermediate product, the charging of the basic intermediate productinto aqueous sulfuric acid, and the heat treatment of the solution ofaluminum chloride sulfate.

According to the invention, aluminum-containing waste products are usedas an aluminum-containing substance. For instance, waste solutions maybe used, which are formed in the chemical industry, e.g., by the etchingof aluminum or in organic syntheses catalyzed by AlCl₃ . It is alsocontemplated according to the invention that the solids-free solutionproduced by the digestion is concentrated by evaporation to an AlCl₃content of about 30 weight percent and an optimum crystallization ofAlCl₃.6H₂O is subsequently effected. In accordance with the invention itis particularly desirable to form an AlCl₃.6H₂O having a median particlediameter d₅₀ between 250 and 450 micrometers and a residual moisturecontent of 3 to 10 weight percent. It will also be particularlydesirable in accordance with the invention to form a solid basicaluminum chloride which has a basicity—i.e., a mole ratio of OH to Al—of 1.35:1 to 2.25:1. In that case the basic aluminum chloride will havea high solubility because 92 to 98 weight percent of that product aresoluble in water or in aqueous sulfuric acid. It is contemplatedaccording to the invention that the basic aluminum chloride is dissolvedin water before it is added to the aqueous sulfuric acid. A particularlygood dissolving behavior will be achieved if, in accordance with theinvention, the solid basic aluminum chloride is dissolved in aqueoussulfuric acid or in water at 40 to 70° C. It is also contemplatedaccording to the invention that the basic solution of aluminum chloridesulfate has an aluminum concentration of 3 to 10 weight percent,preferably 5 to 7.5 weight percent. The solutions prepared by theprocess in accordance with the invention will have particularlydesirable properties if the basic solution of aluminum chloride sulfatehas after the heat treatment a basicity (OH:Al mole ratio) of 1.05:1 to1.95:1 and a sulfate content of 1 to 6 weight percent, preferably 2 to 4weight percent. The properties of the solid basic aluminum chloride willbe particularly constant and good if the AlCl₃.6H₂O is thermallydecomposed in a fluidized bed and it is desirable to form AlCl₃.6H₂Ocrystals having a particle size of 250 to 450 micrometers. Finally, itwill be necessary in some cases to filter the basic solution of aluminumchloride sulfate after the heat treatment in order to remove any solidswhich may be present and which can be recycled in the process as rawmaterials.

The subject matter of the invention will now be explained in detail withreference to examples.

EXAMPLE 1

Alumina hydrate was digested with hydrochloric acid to form a solutionof aluminum chloride. When the insoluble residue had been filtered fromthat solution, the latter was concentrated by evaporation to an AlCl₃content of 30 weight percent and was subsequently supplied at a rate of500 kg/h to a suspension-circulating crystallizer, from which asuspension of crystals was withdrawn, which was supplied to acentrifuge, which delivered a crystallizate that consisted of AlCl₃.6H₂Oand had a residual moisture content of about 6 weight percent. Thatmaterial was subsequently decomposed at 170 to 180° C. in a fluidizedbed reactor until a basicity (OH:Al mole ratio) of 1.8:1 had beenreached. The product withdrawn from the fluidized bed reactor contained3.9 weight percent H₂O and had a median particle diameter d₅₀ of 360micrometers. 87% of the material had a particle diameter in the rangefrom 200 to 500 micrometers. The solid basic aluminum chloride had anexcellent solubility because 99.8 weight percent of one part by weightof the solid could be dissolved in 25 parts by weight water at about100° C. within 10 minutes.

6 kg of the solid basic aluminum chloride were added to and dissolved inaqueous sulfuric acid at 60° C. The resulting basic solution of aluminumchloride sulfate was composed of 5.5 weight percent Al³⁺, 8.7 weightpercent Cl^(−2.9) weight percent SO₄ ²⁻, balance water. The solution wasthen heat-treated at 60° C. with stirring for 2 hours and after the heattreatment the solution had a basicity (OH:Al mole ratio) of about 1.5:1.An insoluble residue amounting to 0.5 weight percent of the solid basicaluminum chloride was filtered from the sulfate-containing solution ofpolyaluminumchloride.

The sulfate-containing basic solution of polyaluminumchloride which hadbeen prepared had a very good stability because it had a turbidity of1.4 TU/F after one day and of 3.7 TU/F after 30 days (TU/F=turbidityunits related to formazine in accordance with DIN 38404 C2).

EXAMPLE 2

A sulfate-containing basic solution of polyaluminumchloride was preparedunder the process conditions stated in Example 1, with the differencethat a solid basic aluminum chloride having a basicity (OH:Al moleratio) of 2.16:1 was formed as an intermediate product in the fluidizedbed. In portions of 6 kg, that solid basic aluminum chloride wasdissolved in different amounts of an aqueous sulfuric acid. Thefollowing sulfate-containing basic solutions of polyaluminumchloridewere formed by a heat treatment at 60° C. for two hours:

Solution 2a: 7.2 weight percent Al³⁺, 9.9 weight percent Cl⁻, 3.9 weightpercent SO₄ ²⁻, basicity (OH:Al mole ratio) 1.65:1.

Solution 2b: 6.0 weight percent Al³⁺, 7.1 weight percent Cl⁻, 3.3 weightpercent SO₄ ²⁻, basicity (OH:Al mole ratio) 1.8:1.

The two sulfate-containing basic solutions of polyaluminumchloride had avery good stability.

EXAMPLE 3

Dilute solutions were prepared from the sulfate-containing basicsolutions 1, 2a and 2b of polyaluminumchloride which had been preparedin Examples 1 and 2 by the process in accordance with the invention. Tothat end, 1 part of solution was mixed with 10 parts of water. Theresulting dilute solutions 1, 2a, and 2b were tested by the so-calledJahr test for their flocculating properties. In theJahr test, 1 ml of asuspension of 5% activated carbon are added to 800 ml water. Thereafterthe flocculation consisting in the present case of the dilute solution1, 2a, or 2b is added in an amount containing 5 mg Al₂O₃ as an activeflocculant to the activated carbon-containing water with stirring at aspeed in excess of 120 revolutions per minute. Thereafter the mixture isstirred at 40 revolutions per minute for 10 minutes. The flocssubsequently settle during a certain settling time and the suspendedactivated carbon is entrained and flocculated. The most importantflocculating properties of the dilute solutions 1, 2a, and 2b are statedin the following table, from which it is apparent that a very goodresidual turbidity can be reached within a short time. Besides, nochanges were exhibited even after 100 days by the solutions used toprepare the dilute solutions 2a and 2b.

TABLE 1 Dilute Solutions 1 2a 2b Floc size (mm) 2.5 3 4 Settling time(minutes) <3 <2 <1.5 Residual turbidity (TU/F) 0.14 0.20 0.15

It has finally been found that sulfate-containing basic solutions ofpolyaluminumchloride having much poorer properties than the products ofthe process in accordance with the invention will be obtained if thestep of dissolving the solid basic aluminum chloride is altered in thatsulfuric acid is charged into an aqueous solution of the basic aluminumchloride and a heat treatment at 60 ° C. is subsequently effected fortwo hours. If in a deviation from the invention Example 1 is carried outin such a manner that the aqueous sulfuric acid is charged into anaqueous solution of the basic aluminum chloride, a sulfate-containingbasic solution of polyaluminumchloride will be obtained, which whendiluted at a ratio of 1:10 has the following poorer flocculatingproperties in the Jahr test:

Floc size 0.3 mm

Settling time >10 minutes

Residual turbidity 0.22 TU/F

What is claimed is:
 1. A process of preparing a solution of basicsulfate-containing polyaluminum chloride, said process comprising thesteps of: a. digesting an aluminum-containing substance withhydrochloric acid to form a resulting solution and an insoluble residue;b. filtering said resulting solution formed in step a) to separate saidinsoluble residue therefrom and to form a solids-free solution; c.concentrating said solids-free solution formed in step b) by evaporationto form a concentrated solution having an AlCl₃ content of about 30percent by weight; d. recovering AlCl₃.6H₂O from said concentratedsolution formed in step c) by crystallization; e. thermally decomposingsaid AlCl₃.6H₂O formed in step d) to form a solid basic aluminumchloride by heating at temperatures from 150 to 200° C. so that saidsolid basic aluminum chloride contains Al and OH in a molar ratio ofOH/Al of from 1.35:1 to 2.25:1; f. feeding the solid basic aluminumchloride formed in step e) into an aqueous sulfuric acid solution anddissolving said solid basic aluminum chloride therein to form anothersolution; and g. subsequently to step f), heat-treating said anothersolution at temperatures of 40° C. to 70° C. for 1 to 3 hours to form asolution of basic sulfate-containing polyaluminum chloride having asulfate content of 1 to 6 percent by weight and an aluminumconcentration of 3 to 10 percent by weight, wherein said molar ratio ofOH/Al in said solution of said basic sulfate-containing polyaluminumchloride is from 1.05:1 to 1.95:1.
 2. The process according to claim 1,wherein said aluminum-containing substance comprises analuminum-containing waste product.
 3. The process according to claim 1,wherein said AlCl₃.6H₂O consists of a plurality of aluminumchloride-containing particles having a mean particle diameter (d₅₀)between 250 and 450 micrometers.
 4. The process according to claim 1,wherein said dissolving of said solid basic aluminum chloride in saidaqueous sulfuric acid takes place at 40 to 70° C.
 5. The processaccording to claim 1, wherein said aluminum concentration of saidsolution of said basic sulfate-containing polyaluminum chloride is 5 to7.5 percent by weight.
 6. The process according to claim 1, wherein saidsulfate content of said solution of said basic sulfate-containingpolyaluminum chloride is 2 to 4 percent by weight.
 7. The processaccording to claim 1, wherein said thermally decomposing of saidAlCl₃.6H₂O occurs in a fluidized bed.
 8. The process according to claim1, further comprising after said heat-treating, filtering said solutionof said basic sulfate-containing polyaluminum chloride.
 9. The processaccording to claim 1, further comprising dissolving said solid basicaluminum chloride in water prior to said feeding into said aqueoussulfuric acid.
 10. The process according to claim 9, wherein saiddissolving of said solid basic aluminum chloride in said water takesplace at 40 to 70° C.